Energy can be obtained from waves by various means, such as utilising the changing wave height, underwater pressure variations, sub-surface fluid particle motion, breading wave effects, and so forth. Many different kinds of devices have been proposed and experimented with during the last two hundred years, but few have achieved even a modest power output or have survived long in the water, because of the difficulty of designing machines which can be built with good output, low material content, high durability and low maintenance requirements, to function reliably in remote locations subject to extreme conditions of exposure to storms and marine corrosion.
"The Development of Wave Power--a Techno-Economic Study", by Leishman and Scobie, published by the National Engineering Laboratory, East Kilbride, Scotland, in 1975, lists British Patents on wave-powered devices from 1855, and illustrates typical concepts classified according to their means of operation, together with a list of organisations in various countries studying wave power, usually for the generation of electricity. Subsequent researchers up to the present time investigated various ideas (most of which are described in "Offshore Engineer--January 1981") and have participated with the Japanese in a full-scale trial at sea (the "Kaimei" barge experiment).
Systems which have merited serious study have generally, apart from a Swedish buoy example with mechanical power linkage, fallen into two groups, utilising either hydraulic or pneumatic intermediate stages to convert wave energy into motive power. Of the former, the "Salter Duck" and the "Cockerell Raft" have been demonstrated at reduced scale. Both proposals utilised hydraulic power generated by the opposition of hinged floats following the wave profile, but failed to attain reasonable cost/output ratios and have been abandoned. A generally-favoured, but questionable view today is that pheumatic conversion machines, which use wave motion to force air through turbines, are the most practical. This opinion is largely based on the success of small air-turbine-powered navigation light buoys produced by the Japanese Ryokuseisha Corporation, but ignores the failure of the Japanese "Kaimei" barge experiment which utilised air turbines over "wave-piston" air compartments.
Other proposed machines based on similar air displacement principles include the "Lancaster Bag", the National Engineering Laboratory's "Oscillating Water Column", the "Vickers Device", and the "Lanchester Clam". Disadvantages of these devices utilising air-driven turbines are:
(1) Low mass and low kinetic energy of the air fluid flow to the turbines.
(2) Flow pressure to an air turbine of given size depends only on the incident wave magnitude--it does not increase with air chamber or bellows volume. Consequently, the larger the air compartment, the greater the mechanical disadvantage and the more adverse the cost/benefit ratio.
(3) The compressibility of the air fluid medium absorbs and reduces input energy.
(4) Pulsating or reciprocating air flow leads to unavoidable inertia losses.
(5) Because of these inherent drawbacks, air-driven machines will only function effectively within narrow design limits.
(6) Having substantial mass and strong compartments in order to withstand wave impact, they tent to be large and costly structures in relation to the power generated.